The present invention relates to a method for thermal treatment of metal workpieces in a gas atmosphere containing nitrogen, in particular for nitrocarburizing iron articles. The invention furthermore relates to the use of an apparatus for performing such a method.
Metal workpieces are subjected to a thermochemical heat treatment for producing defined workpiece properties, e.g. high resistance to wear or sufficient corrosion resistance. In nitriding and nitrocarburizing, the result of the heat treatment is that the case of the workpiece is enriched with nitrogen and/or carbon in order to provide the workpiece with the required mechanical and chemical properties at the surface and in the case.
In nitriding, e.g. in a gas atmosphere containing ammonia, the surface layer or case is enriched with nitrogen in that the ammonia (NH3) contained in a reaction gas generally breaks down into nitrogen (N) and hydrogen (H) at temperatures greater than 500xc2x0 C. under the catalytic effects of the surface of the workpieces that are to be subjected to nitriding. The ammonia molecule is adsorbed and gradually broken down at the workpiece surface, whereby the required nitrogen is released in its atomic form and is available for dissolving in the iron and for forming iron nitride (FexRN). In nitrocarburizing, in addition, the case is simultaneously enriched with carbon. Atomic carbon (C) diffuses through the surface of the workpiece into the case in an analogous manner.
The outermost region of the case, the so-called connecting or white layer, is of particular importance in terms of the properties that the treated workpiece must have. It is generally between 1 xcexcm and 30 xcexcm in thickness, and in nitriding or nitrocarburizing it comprises primarily hexagonal xcex5-nitride, (Fe2-3N) and cubic face-centered xcex3xe2x80x2-nitride (Fe4N). Among processing parameters, the temperature and treatment duration selected impact the properties of the connecting layer, but the composition of the reaction gas used has the greatest impact. This is because the amount of the elements diffusing through the surface into the case (nitrogen (N), carbon (C), and even oxygen (O) and sulfur (S)) is determined by the composition of the reaction gas at given temperatures and treatment durations.
The nitride coefficient for the reaction gas KN=pNH3/pH23/2, determined from the quotient of the partial pressure of ammonia (pNH3) and the 1.5xc3x97 potency of the partial pressure of hydrogen (pH23/2), and the carburizing coefficient for the reaction gas Kc=pCO2/pCO2, determined, e.g. from the quotient of the square of the partial pressure of carbon monoxide (pCO2) and the partial pressure of carbon dioxide (pCO2) or alternatively determined from the quotient for the partial pressure of methane (pCH4) and the square of the partial pressure of hydrogen (pH22), are key to the content of nitrogen and carbon in the connecting layer, which connecting layer is produced primarily depending on the process parameters of temperature and treatment duration and furthermore depending on the composition of the reaction gas. Thus, given a reaction gas for nitrocarburizing, the content of which has been optimized in terms of xcex5-nitride, as is conventionally done, and the composition of which is 50 vol. % ammonia (NH3) and 50 vol. % endothermic gas, the carburizing coefficient KC is between 1.5 and 2.5 when the amount of ammonia in the gas atmosphere that is converted during nitrocarburizing is between 15 vol. % and 40 vol. %. In contrast, the carburizing coefficient KC in a reaction gas with a composition of 50 vol. % NH3, 45 vol. % N2, and 5 vol. % CO2 is substantially lower.
If nitrocarburizing is performed with an amount of carbon dioxide in the gas atmosphere that is between 0 vol. % and 7 vol. % and an amount of ammonia that is between 0 vol. % and 40 vol. %, the value for the carburizing coefficient KC increases between 0 and 0.5. The nitride coefficient KN and the carburizing coefficient KC are mutually dependant due to the balance of the components carbon monoxide (CO), water vapor (H2O)V, carbon dioxide (CO2), and hydrogen (H2) in the gas atmosphere, as described by the formula:       CO    +                  H        2            ⁢      O        ⇆            CO      2        +          H      2      
The result of this is that the carburizing coefficient KC, at a predetermined nitride coefficient KN, cannot be changed except in a limited measure and thus is of limited utility for influencing workpiece properties. It is furthermore disadvantageous that the carburizing coefficient KC is not high enough in the conventionally used reaction gases so that technical properties of metal workpieces that are influenced by the content of carbon in the connecting layer, e.g. resistance to wear or resistance to corrosion, cannot be fully exploited.
It is therefore an object of the present invention to further develop a method for the thermal treatment of metal workpieces such that it is possible to obtain enhanced resistance to wear and corrosion in the treated workpieces.